During imaging chip design, a thin top metal or conducting layer guard ring surrounding the pixel perimeter is included. The purpose of this ring is to act as a barrier to charges leaking to or from the pixel. Conventionally, this ring is tied to a fixed voltage or ground potential and, therefore, pixel leakage current normally flows to or from this ring and not to the adjacent pixels in the circuit. The leakage current is proportional to the pixel to ring voltage difference and inversely proportional to the equivalent leakage resistance. One conventional technique to reduce surface leakage current to acceptable limits is to increase the surface leakage resistance. This increase often requires super cleaning the surface of the image chip. To further reduce the surface leakage current, post processing of the image chip is utilized to form a lengthy path for possible leakage charges, thereby causing an increase in effective surface leakage resistance, often at the expense of chip yield and cost.
As will be explained, the present invention provides an active guard ring that acts as a barrier to reduce or eliminate charge leakage between pixels. By controlling the potential of the active guard ring to be near the potential of the pixel, it is possible to reduce or eliminate leakage current.